This invention relates generally to gas turbine engines, and more particularly, to methods and apparatus for assembling gas turbine engines.
Known gas turbine engines include a combustor which ignites a fuel-air mixture that is then channeled through a turbine nozzle assembly to power a turbine. Known turbines include a plurality of turbine rotor blades surrounded by a circumferential turbine shroud assembly. The combustion exit gases are channeled through the turbine nozzle assembly and directed towards the rotor blades to cause rotation of the turbine.
At least some known turbine nozzle assemblies include a plurality of circumferentially-oriented nozzle segments. Known turbine nozzle segments are fabricated with at least two circumferentially-spaced hollow airfoil vanes coupled together by integrally-formed inner and outer band platforms. The inner band defines a portion of the radially inner flowpath boundary and the outer band defines a portion of the radially outer flowpath boundary.
As relatively high temperature combustion gases are channeled through the turbine nozzle assembly, over time, the high temperatures may cause the turbine nozzle assembly and the turbine shroud to oxidize. Because of their orientation relative to the gas flow, an inner surface and a rear face of the turbine nozzle assembly outer band are generally most susceptible to oxidation. Moreover, in at least some known turbine nozzle assemblies, oxidation may occur in a discrete arc extending along a throat area defined between adjacent airfoil vanes, wherein combustion gases are channeled through the turbine nozzle assemblies. In at least some other known turbine nozzle assemblies, oxidation may occur along the rear face of the outer band as combustion gas are channeled through a gap defined between the nozzle assembly and the turbine shroud, a condition known as gas path ingestion.
In at least some known gas turbine engines, additional cooling air is channeled to each turbine component to facilitate reducing an operating temperature of the component to yield an acceptable rate of oxidation. However, increasing the flow of cooling air increases the overall operating costs of the engine. Specifically, the increased cooling air may increase the specific fuel consumption of the engine, thus increasing the overall operating costs of the engine.